ARTICLES NATURE METHODS | VOL.8 NO.4 | APRIL 2011 | 319 Recording electrical activity from identified neurons in intact tissue is key to understanding their role in information processing. Recent fluorescence labeling techniques have opened new possibilities to combine electrophysiological recording with optical detection of individual neurons deep in brain tissue. For this purpose we developed dual-core fiberoptics–based microprobes, with an optical core to locally excite and collect fluorescence, and an electrolyte-filled hollow core for extracellular single unit electrophysiology. This design provides microprobes with tips <10 mm, enabling analyses with single-cell optical resolution. We demonstrate combined electrical and optical detection of single fluorescent neurons in rats and mice. We combined electrical recordings and optical Ca 2+ measurements from single thalamic relay neurons in rats, and achieved detection and activation of single channelrhodopsin-expressing neurons in Thy1::ChR2-YFP transgenic mice. The microprobe expands possibilities for in vivo electrophysiological recording, providing parallel access to single-cell optical monitoring and control. The central nervous system is characterized by heterogeneous populations of cells with highly specialized phenotypes defined by their morphological, biochemical and physiological proper- ties. Efficient characterization of each cell type in intact tissue has remained challenging because the neurons of interest are often dispersed in these heterogeneous populations. There are several approaches that allow electrophysiologists to identify the cell types they record from, but most of these approaches have drawbacks. Antidromic activation of projection neurons is only applicable to a small subset of cells and requires preservation of connections between the recording and projection sites 1,2 . Labeling cells with dyes at the time of recording allows a posteriori identification, but this approach is inefficient when dealing with subpopulations that represent a small proportion of the overall neuronal population in an area 3,4 . Labeling with fluorescent markers allows for targeted recording, independently of connectivity 5–9 and has been very instrumental for studies in central nervous system tissue slices 10–12 . However, it cannot be fully exploited for electrophysiological investigations in live animals because light scattering limits optical microscopy to surface measurements (500–1,000 µm) 13,14 . Micro- endoscopes help overcome limitations for deep tissue imaging but do not allow for combined electrical and optical monitoring, especially for single unit recording 15,16 . The challenge remains to conduct combined electrophysiological recording and optical identification of individual neurons deep into central nervous system tissue in live animals. Combination of both approaches is important because purely optical techniques remain limited for resolving single action potentials and for prolonged functional measurements (>1 h). Several strategies have been sought to combine microelec- trode and guided optical recording. One had used distinct probes brought together using two independent microdrives 17,18 . Such an approach is not practical for in vivo measurements. Other groups have relied on attaching large fibers (250 µm in diameter) to an electrode 19,20 . Even with smaller optical fibers (25–35 µm) inserted into a micropipette, the final tip diameter of the probe is 35–45 µm, precluding combined optical and electrical record- ing from a single neuron. Furthermore, this approach relies on an external source of illumination, which prevents efficient light delivery into deep tissue and produces tissue excitation over wide areas 21 . To overcome these limitations, we designed a new type of opti- cal fiber, incorporating both an optical and a hollow core. We show that this type of microprobe provides, to our knowledge for the first time, sufficient spatial resolution to correlate electro- physiological and fluorescence signals that emanate from single fluorescently labeled neurons at a depth of >6,000 µm in the intact central nervous system. RESULTS The microprobe Our objective was to detect, in parallel, the electrical field and the optical signal from single fluorescent neurons in vivo. We designed an optical fiber composed of a hollow core and an opti- cal core (Fig. 1a,b and Supplementary Fig. 1a). We filled the hollow core with an electrolyte solution (1–3 M NaCl) to record 1 Centre de recherche Université Laval Robert-Giffard, Québec, Canada. 2 Centre d’optique, photonique et laser, Université Laval, Québec, Canada. 3 Department of Psychiatry and Neuroscience, Université Laval, Québec, Canada. 4 These authors contributed equally to this work. Correspondence should be addressed to Y.D.K. (yves.dekoninck@crulrg.ulaval.ca). RECEIVED 15 JULY 2010; ACCEPTED 20 JANUARY 2011; PUBLISHED ONLINE 13 FEBRUARY 2011; DOI:10.1038/NMETH.1572 A microprobe for parallel optical and electrical recordings from single neurons in vivo Yoan LeChasseur 1,2 , Suzie Dufour 1,2,4 , Guillaume Lavertu 1,4 , Cyril Bories 1 , Martin Deschênes 1,3 , Réal Vallée 1,2 & Yves De Koninck 1–3 © 2011 Nature America, Inc. All rights reserved.